1. Field of the Invention
The present invention relates to a video camera for converting an object image imaged by a photographing optical system into an electrical signal using photoelectric conversion means.
2. Description of the Related Art
In recent years, a video camera for converting an object image imaged by a photographing optical system into an electrical signal using a photoelectric conversion element such as a solid-stage image pickup element, e.g., a CCD (charge coupled device), and recording the electrical signal onto a recording medium such as a magnetic tape has become popular. To facilitate ease in use, video cameras are required to have compact structures, and compact video cameras have been developed by means of high-density electrical circuit packages, compact CCDs, compact photographing optical systems, and the like. At the present time, extensive studies continue to be made to realize development of further compact video cameras. For this reason, a further compact photographing optical system is desirable.
The amount of light incident on an image pickup unit of a video camera is adjusted as follows. That is, an iris, which can mechanically adjust its aperture size, is arranged in a photographing optical system, and the aperture size of the iris is adjusted.
However, since such a mechanical iris unit has a large driving motor unit, it is large in comparison to a photographing optical system lens barrel with a compact CCD. This problem frustrates the realization of a compact photographing optical system lens barrel.
It has been proposed that a material element such as a liquid crystal element, an EC (electrochromic) element, or the like be used as an iris or a variable ND (neutral density) filter in place of the mechanical iris unit. However, these material elements still suffer from problems associated with wavelength dependency (spectral transmission factor) of a transmission factor, a small light transmission factor in a complete transmission state, a very narrow transmission factor adjustment range as compared to the above-mentioned mechanical iris, and the like. For these reasons, the material element has not yet been put into practical applications as an iris or a variable ND filter for a photographing optical system.